DE102017002735B4 - Cordierite-based sintered body, method for producing the same and use - Google Patents

Abstract

A cordierite-based sintered body, the cordierite-based sintered body comprising:
a coefficient of thermal expansion of less than 2.4 ppm/°C at 40°C to 400°C,
an open porosity of 0.5% or less, and
an average grain size of sintered grains of 1 µm or less,
wherein the cordierite-based sintered body contains silicon nitride, and
in the case where a polished surface is subjected to observation from a backscattered electron image and compositional analysis with an SEM, and then an area ratio of a cordierite phase to a silicon nitride phase is determined from a contrast ratio of the image, and
is converted to a volume ratio of the sintered body, the cordierite phase is 60% to 90% by volume, and the silicon nitride phase is 10% to 40% by volume.

Description

The present invention relates to a cordierite-based sintered body, a method for producing the cordierite-based sintered body, and the use of the sintered body in a composite substrate.

Cordierite is a material with high heat resistance and a low coefficient of thermal expansion, and thus is known as a material exhibiting high thermal shock resistance. To improve the mechanical properties of cordierite, it is also known to add silicon nitride or silicon carbide, which have high Young's modulus and high strength (PTL 1 and 2). In PTL 1, a cordierite-based sintered body having a relative density of 97% to 98% is prepared by adding a rare earth oxide and silicon nitride or silicon carbide to cordierite having an average particle diameter of 1.2 μm and firing the mixture in air. In PTL 2, a cordierite-based sintered body having a high Young's modulus is produced by adding silicon nitride or silicon carbide having an average particle diameter of 1 µm to cordierite having a particle diameter of 3 µm and firing the mixture in a nitrogen atmosphere at normal pressure.

PTL 3 discloses an example in which a composite substrate containing a functional substrate bonded directly to a support substrate, the functional substrate being composed of lithium tantalate, lithium niobate or the like, and the support substrate being composed of a cordierite-based sintered body, for an acoustic wave device such as a surface acoustic wave element is used. In the acoustic wave device, the dependency of frequency on temperature is significantly improved because the cordierite-based sintered body serving as the supporting substrate has a very small coefficient of thermal expansion of about 1.1 ppm/°C (40°C to 400°C). having. PTL 4 relates to a silicon nitride cordierite ceramic product and a method of making the same. In addition, PTL 5 discloses a cordierite-based dense sintered body and a method for producing the same.

• PTL 1: JP 3574560 B2
• PTL 2: JP 4416191 B2
• PTL 3: WO 2015/186571 A1
• PTL 4: EP 0 152 481 B1
• PTL 5: EP 1 582 509 A2

When the functional substrate and the support substrate are bonded together, as in PTL 3, both substrates are required to have a high degree of surface flatness. However, because the cordierite-based sintered body described in PTL 1 has a low relative density of 97% to 98% and a porosity of a few percent, even if the surface of the sintered body is subjected to polishing finishing, a high degree of flatness is not obtained . In the cordierite-based sintered body described in PTL 2, because the raw material cordierite particles have a large particle diameter of 3 µm and the addition of a sintering aid, the size of the sintered grains is larger than that of the raw material cordierite particles. Thus, a high degree of flatness is not obtained even if polishing finishing is carried out. When such a cordierite-based sintered body is used for an acoustic wave device, the size of sintered grains may be larger than an interelectrode gap. In this case, variations in the acoustic velocity of the composite substrate can take place to cause variations in device characteristics.

The present invention has been made to solve these problems, and mainly aims at providing a cordierite-based sintered body with high rigidity while maintaining the low thermal expansion coefficient of cordierite and a high degree of flatness of a mirror-finished surface of the cordierite-based sintered body.

A cordierite-based sintered body according to the present invention has a thermal expansion coefficient of less than 2.4 ppm/°C at 40°C to 400°C, an open porosity of 0.5% or less, and an average grain size of sintered grains of 1 μm or less up,
wherein the cordierite-based sintered body contains silicon nitride, and in the case where a polished surface is subjected to observation from a backscattered electron image and composition analysis with an SEM, and then an area ratio of a cordierite phase to a silicon nitride phase is determined from a contrast ratio of the image and to a volume ratio of the sintered body is converted, the cordierite phase is 60% to 90% by volume and the silicon nitride phase is 10% to 40% by volume; or
wherein the cordierite-based sintered body contains silicon carbide, and in the case where a polished surface is subjected to observation from a backscattered electron image and composition analysis with an SEM, and then an area ratio of a cordierite phase to a silicon carbide phase is determined from a contrast ratio of the image and to a Volume ratio of the sintered body is converted, the cordierite phase is 70% to 90% by volume and the silicon carbide phase is 10% to 30% by volume. The cordierite-based sintered body has high rigidity while maintaining the low coefficient of thermal expansion of cordierite and a high degree of flatness of a mirror-finished surface thereof.

A method for producing a cordierite-based sintered body according to the present invention includes the steps of (a) preparing a raw material powder mixture by mixing 60% to 90% by volume of a cordierite powder having an average particle diameter of 0.1 to 1 μm and 10% to 40% by volume of a silicon nitride powder having an average particle diameter of 0.1 to 1 µm in a total amount of 100% by volume, or by mixing 70% to 90% by volume of a cordierite powder having an average particle diameter of 0.1 to 1 µm and 10% to 30% by volume of a silicon carbide powder having an average particle diameter of 0.1 to 1 µm in a total amount of 100% by volume; and (b) preparing a cordierite-based sintered body by forming the raw material powder mixture into a compact having a predetermined shape and subjecting the compact to hot pressing at a compacting pressure of 20 to 300 kgf/cm ² and a firing temperature of 1350°C to 1450°C (maximum temperature). This manufacturing method is suitable for manufacturing the foregoing cordierite-based sintered body according to the present invention. The mean particle diameter of each of the powders is a value measured by laser diffractometry (the same is applied hereinafter).

• 1 veranschaulicht Schritte bei der Herstellung von einem Cordierit-basierten Sinterkörper.
• 2 ist eine perspektivische Ansicht eines Verbundwerkstoffsubstrats 10.
• 3 ist eine perspektivische Ansicht einer elektronischen Vorrichtung 30, hergestellt durch die Verwendung des Verbundwerkstoffsubstrats 10.
• 4 veranschaulicht SEM-Bilder von einer Hochglanz-polierten Oberfläche von einem Cordierit-basierten Sinterkörper in Versuchs-Beispiel 3, (a) durch Erläutern von Rohdaten und (b) durch Erläutern binärisierter Daten.

A composite substrate according to the present invention includes a functional substrate and a support substrate bonded to the functional substrate, the support substrate being composed of the foregoing cordierite-based sintered body. In the composite substrate, satisfactory bonding to the functional substrate is achieved due to a high degree of flatness from a mirror-polished surface of the cordierite-based sintered body serving as the support substrate. When the composite substrate is used for a surface acoustic wave element, the dependence of frequency on temperature is remarkably improved. For an optical waveguide device, an LED device, and a switching device, performance is improved due to a low thermal expansion coefficient of the support substrate.

• 1 illustrates steps in manufacturing a cordierite-based sintered body.
• 2 12 is a perspective view of a composite substrate 10.
• 3 12 is a perspective view of an electronic device 30 manufactured by using the composite substrate 10.
• 4 12 illustrates SEM images of a mirror-polished surface of a cordierite-based sintered body in Experimental Example 3, (a) by explaining raw data and (b) by explaining binarized data.

The cordierite-based sintered body of one embodiment of the present invention contains cordierite as a main component and silicon nitride or silicon carbide. The term "major component" refers to a component that contributes to the largest volume of the sintered body. The cordierite-based sintered body has a thermal expansion coefficient of less than 2.4 ppm/°C at 40°C to 400°C, an open porosity of 0.5% or less, and an average grain size of sintered bodies of 1 μm or less. Although the cordierite-based sintered body contains silicon nitride or silicon carbide having a higher coefficient of thermal expansion than cordierite, the cordierite-based sintered body maintains a low coefficient of thermal expansion. The cordierite-based sintered body contains silicon nitride or silicon carbide having a higher Young's modulus than cordierite and thus has high rigidity as compared with cordierite alone. Because the cordierite-based sintered body has an open porosity of 0.5% or less, few pores, and a small mean grain size of sintered bodies of 1 µm or less, a surface subjected to polishing finishing (mirror-polished surface) has a high degree of flatness.

In the cordierite-based sintered body of the present embodiment, the number of pores that have a maximum length of 1 μm or more and that are present on a mirror-finished surface with an area of 100 μm×100 μm is preferably 10 or less. When the number of pores is 10 or less, the surface subjected to polishing finishing has a higher degree of flatness. The number of pores is preferably three or less, and more preferably zero.

The cordierite-based sintered body of the present embodiment preferably has a Young's modulus of 160 GPa or more, and preferably has a four-point bending strength of 220 MPa or more. Because silicon nitride and silicon carbide have higher Young's modulus and strength than cordierite, the cordierite-based sintered body can have a Young's modulus of 160 GPa or more and a four-point bending strength of 220 MPa or more by adjusting the ratio from silicon nitride or silicon carbide to cordierite.

In the cordierite-based sintered body of the present embodiment, a mirror-finished surface preferably has a center-line average roughness Ra of 1.5 nm or less. A composite substrate in which a functional substrate is bonded to a supporting substrate is known as a composite substrate used for an acoustic wave device. When the cordierite-based sintered body having a mirror-polished surface with a center-line average roughness Ra of 1.5 nm or less as described above is used as the supporting substrate, satisfactory bondability between the supporting substrate and the functional substrate is obtained. For example, the area percentage of an actually bonded portion of a bonded interface (bonding area percentage) is 80% or more (preferably 90% or more). The center line average roughness Ra of the mirror-finished surface is preferably 1.1 nm or less, more preferably 1.0 nm or less, and particularly preferably 0.8 nm or less.

The cordierite-based sintered body of the present embodiment more preferably has a thermal expansion coefficient of 2.0 ppm/°C or less at 40°C to 400°C. In the case when a composite substrate containing the cordierite-based sintered body serving as a support substrate is used for an acoustic wave device, the dependence of the frequency of the acoustic wave device on temperature is improved because the amount of thermal expansion of the functional substrate is smaller than the original amount of thermal expansion when the temperature of the acoustic wave device increases. The coefficient of thermal expansion is more preferably 1.8 ppm/°C or less at 40°C to 400°C.

When the cordierite-based sintered body of the present embodiment contains silicon nitride, a cordierite phase is 60% to 90% by volume and a silicon nitride phase is 10% to 40% by volume. When the cordierite-based sintered body of the present embodiment contains silicon carbide, the cordierite phase is 70% to 90% by volume and a silicon carbide phase is 10% to 30% by volume. At these compositional ratios, properties such as the number of pores, Young's modulus, center line average roughness Ra, and thermal expansion coefficient at 40°C to 400°C are satisfactory. The volume % of each phase is determined as follows: A polished surface of the cordierite-based sintered body of the present embodiment is subjected to observation from a backscattered electron image and composition analysis with an SEM. The area ratio of the phases determined from a contrast ratio of the image is converted to a volume ratio (volume %) of the phase of the sintered body.

An embodiment of a method for producing a cordierite-based sintered body according to the present invention will be described below. As in 1 explained, a method for producing a cordierite-based sintered body includes the steps of (a) preparing a raw material powder mixture and (b) preparing a cordierite-based sintered body.

· Step (a): Preparation of raw material powder mixture

As a cordierite raw material, a powder having a high purity and a small average particle diameter is preferably used. The purity is preferably 99.0% or more, more preferably 99.5% or more, and still more preferably 99.8% or more. The units of purity are % by mass. The average particle diameter (D50) is preferably 1 µm or less, and more preferably 0.1 to 1 µm. As the cordierite raw material, a commercial example or a material made of high-purity powders of magnesia, alumina and silica can be used. An example of a method for producing a cordierite raw material is a method described in PTL 3. As a silicon A nitride raw material and a silicon carbide raw material are preferably used powders having a small average particle diameter. The average particle diameter is preferably 1 µm or less, and more preferably 0.1 to 1 µm. In the case where a raw material powder mixture of the cordierite raw material and the silicon nitride raw material is prepared, for example, the raw material powder mixture can be prepared by weighing 60% to 90% by volume of the cordierite raw material and 10% to 40% by volume of the silicon nitride raw material in a total amount of 100% by volume, performing mixing with a mixer such as a pot mill, and if necessary, performing drying with a spray dryer. In the case where a raw material powder mixture of the cordierite raw material and the silicon carbide raw material is prepared, for example, the raw material powder mixture can be prepared by weighing 70% to 90% by volume of the cordierite raw material and 10% to 30% by volume of the silicon carbide raw material in a total amount of 100% by volume, performing mixing with a mixer such as a pot mill, and if necessary, performing drying with a spray dryer.

· Step (b): Production of cordierite-based sintered body

The raw material powder mixture prepared in the step (a) is formed into a compact having a predetermined shape. A forming method is not particularly limited, and an ordinary forming method can be used. For example, the raw material powder mixture can be press-molded directly with a die. When the raw material powder mixture is spray dried into granules, satisfactory moldability is obtained in the case of press molding. A green body or a suspension prepared by adding an organic binder may be subjected to extrusion molding or sheet molding. In this process, the organic binder component must be removed before or during a firing step. High pressure forming can be accomplished by cold isostatic pressing (CIP).

The obtained compact is fired to form a cordierite-based sintered body. At this time, maintaining fine sintered grains and removing pores during firing are preferable for improving the degree of surface flatness of the cordierite-based sintered body. As a method for this, a hot press method is very effective. The use of the hot-press method allows densification to proceed in a fine grain state, kept at a lower temperature than that in pressureless sintering, and suppresses the formation of coarse pores, which are frequently observed in pressureless sintering. The firing temperature in the hot pressing is preferably 1350°C to 1450°C, and more preferably 1375°C to 1425°C. The pressing pressure in the hot pressing is preferably 20 to 300 kgf/cm ² . In particular, a lower pressing pressure results in a hot-press jig having a smaller size and a longer service life, and is thus preferable. The dwell time at the firing temperature (maximum temperature) can be appropriately selected in view of the shape and size of the molded article, properties of a heating furnace, and so on.

Specifically, the residence time is preferably 1 to 12 hours, and more preferably 2 to 8 hours. A firing atmosphere is not particularly limited. An atmosphere during hot pressing is generally an inert atmosphere such as nitrogen or argon. The rate of temperature increase and the rate of temperature decrease can be set appropriately in view of the shape and size of the molded body, properties of the heating furnace and so on, and can be set in the range of 50 to 300°C/h, for example.

A composite substrate according to an embodiment of the present invention will be described below. The composite substrate of the present embodiment includes a functional substrate bonded to a supporting substrate composed of the cordierite-based sintered body. In this composite substrate, the high area percentage of bonded portion between both substrates provides satisfactory bondability. Examples of the functional substrate include, but are not particularly limited to, lithium tantalate, lithium niobate, gallium nitride, and silicon. A binding method is preferably direct binding. In the case of direct bonding, after bonding, the surfaces of the functional substrate and the supporting substrate are mirror-polished and activated, both substrates are pressed together with both bonding surfaces facing each other. The activation of the bonding surfaces is carried out by, for example, irradiating the bonding surface with an ion beam of an inert gas (such as argon) or with a plasma or neutral atom beam. The thickness ratio of the functional substrate to the supporting substrate (thickness of functional substrate/thickness of supporting substrate) is preferably 0.1 or less. 2 illustrates an example of a composite substrate. The composite substrate 10 is a Substrate in which a piezoelectric substrate 12 serving as a functional substrate is bonded to a supporting substrate 14 by the direct bonding.

The composite substrate of the present embodiment is applicable to electronic devices and so on. Examples of electronic devices include acoustic wave devices (such as surface acoustic wave elements, Lamb wave devices, and film bulk acoustic resonators (FBARs)), LED devices, optical waveguide devices, and switching devices. When the foregoing composite substrate is used for an acoustic wave device, it is because the cordierite-based sintered body serving as a support substrate has a very small coefficient of thermal expansion of less than 2.4 ppm/°C (40°C to 400°C). , the dependence of frequency on temperature is significantly improved. 3 12 explains an example of the electronic device 30 manufactured by using the composite substrate 10. The electronic device 30 is a one-port SAW resonator or a surface acoustic wave element. Many patterns each used for the electronic device 30 are formed on the piezoelectric substrate 12 of the composite substrate 10 by ordinary photolithography technology and then cut into the individual electronic devices 30 by dicing. Each of the electronic devices 30 is a device in which interdigital transducer (IDT) electrodes 32 and 34 and reflector electrodes 36 are formed on a surface of the piezoelectric substrate 12 by photolithography technology.

It should be understood that the present invention is not limited to the foregoing embodiments and various modifications can be made without departing from the technical scope of the present invention.

examples

1. Preparation of raw material powder mixture

A cordierite raw material was prepared from commercially available high-purity magnesia, alumina and silica powders having an average particle diameter of 1 µm or less and a purity of 99.9% or more. Specifically, each of the powders was weighed and mixed so as to obtain the composition of cordierite. The mixture was heated in an air atmosphere at 1400°C for 5 hours to form cordierite coarse particles. The obtained cordierite coarse particles were milled with a pot mill containing alumina serving as crushing media (diameter: 3 mm) and deionized water serving as a solvent for 70 hours to form a pulverized cordierite suspension having an average particle diameter of pulverized about 0.5 to 0.6 µm. The suspension obtained was dried in air at 110°C. The dry matter was passed through a sieve to provide a cordierite powder. The cordierite raw material and a silicon nitride raw material or a silicon carbide raw material were weighed to achieve the raw material composition of each of Experimental Examples 1 to 9 in Table 1, and then together using a pot mill containing alumina grinding media with a diameter of 5 mm. The obtained mixture was spray-dried to prepare a raw material powder mixture. As the silicon nitride raw material, a commercially available powder having an average particle diameter of 0.8 μm and a purity of 97% or more was used. As the silicon carbide raw material, a commercially available powder having an average particle diameter of 0.5 μm and a purity of 97% or more was used.

2. Production of cordierite-based sintered body

The raw material powder mixtures of Experimental Examples 1 to 9 were each pressurized at 50 kgf/cm ² by uniaxial compression molding to form a green compact having a diameter of 100 mm and a thickness of about 25 mm. The green compact was placed in a graphite die and heated with a hot-press heating furnace at a firing temperature (maximum temperature) of 1375°C to 1425°C for 5 hours at a pressing pressure of 200 kgf/cm ² to produce a cordierite-based one sintered body fired. The firing temperature of each experimental example is listed in Table 1. The firing atmosphere was an argon atmosphere. The temperature rise rate was 100°C/h. The rate of temperature decrease was 200°C/h. Furnace cooling was carried out at 1200°C or lower during cooling. In Experimental Example 10, a compact was formed and fired in a manner similar to the cordierite powder alone, and heated with the hot-press heating furnace at a compacting pressure of 200 kgf/cm ² and a firing temperature (maximum temperature) of 1425° C. for 5 hours Making a sintered body composed of cordierite alone fired.

3. Evaluation of properties

Test pieces (for example, 4×3×40 mm bending bars) were cut from the cordierite-based sintered bodies of Experimental Examples 1 to 10, and subjected to evaluation tests. A surface of each of the approximately 4 × 3 × 10 mm test pieces was mirror-finished by polishing and as a high mirror-polished surface of a corresponding one of the sintered body is used. Polishing was carried out as follows: lapping was carried out with diamond burs of 3 µm, diamond burs of 0.5 µm and, as finishing, diamond burs of 0.1 µm or less. The properties that were evaluated are described below.

(1) Crystal phase

The sintered bodies were pulverized, and then the crystalline phases were identified with an X-ray spectrograph. The measurement conditions were as follows: CuKα radiation, 50 kV, 300 mA and 20 = 5° to 70°. A rotating anode X-ray spectrograph (RINT, manufactured by Rigaku Corporation) was used.

(2) Composition of sintered body

The mirror-finished surface of each of the sintered bodies finished as described above was subjected to observation of a backscattered electron image and compositional analysis with an SEM. The area ratio of a cordierite phase to another phase was determined from the contrast ratio of the image and converted to a volume ratio of the sintered body. Examples of SEM images of the mirror-polished surface are given in 4 explained. 4 FIG. 12 illustrates SEM photographs of the mirror-polished surface of the cordierite-based sintered body of Experimental Example 3 (a) by explaining the raw data and (b) by explaining the binarized data. In 4(a) dark portions indicate the cordierite phase and whitish portions indicate the silicon nitride phase.

(3) Bulk density and open porosity

Bulk density and open porosity were measured with the flex bars by the Archimedes method with deionized water.

(4) Relative density

The calculated density of each of the sintered bodies was calculated from the composition of the sintered body and the densities of the components, and the percentage of the bulk density measured as described above with respect to the calculated density was defined as a relative density. Here, the density of cordierite was 2.505 g/cm ³ , the density of silicon nitride was 3.20 g/cm ³ , and the density of silicon carbide was 3.21 g/cm ³ . The densities of silicon nitride and silicon carbide used here are values obtained by ignoring the effect of impure oxygen and so on in the raw materials.

(5) Flexural strength

The four-point bending strength was measured in accordance with JIS R1601. Test pieces were 3mm x 4mm x 40mm flexbars or flexbars half the size thereof.

(6) Young's modulus

Young's modulus was measured by a static deflection method in accordance with JIS R1602. Test pieces were 3mm x 4mm x 40mm flexure bars.

(7) Coefficient of thermal expansion (40°C to 400°C)

The coefficient of thermal expansion was measured by using a differential type bumper in accordance with JIS R1618. Test pieces were 3 mm × 4 mm × 20 mm in size.

(8) number of pores

The mirror-finished surface of each of the sintered bodies finished as described above was observed with an SEM to measure the number of pores having a maximum length of 1 µm or more in a 100 µm × 100 µm area.

(9) Surface Flatness (Ra)

The center line average roughness Ra of the mirror-finished surface of each of the sintered bodies finished as described above was measured with an AFM. The measurement area was a 10 µm × 10 µm area.

(10) Mean grain size of sintered grain

The mirror-polished surface of each of the sintered bodies finished as described above was thermally etched at 1200° C. to 1400° C. for 2 hours. The size of 200 or more grains of the sintered grains was measured with an SEM, and the mean grain size thereof was calculated using linear analysis. The linear analysis coefficient was 1.5. A value obtained by multiplying the actually measured length by the SEM by 1.5 was defined as the average grain size.

(11) Bonding Ability

Discs having a diameter of about 100 mm and a thickness of about 600 µm were cut from the sintered bodies of Experimental Examples 1-10. The discs were given a polishing finish as described above and washed to remove particles and contaminants on their surfaces. Each of the discs was used as a supporting substrate, and the supporting substrate and a functional substrate were bonded directly to form a composite substrate. Specifically, the bonding surfaces of the support substrate and the functional substrate were activated with an argon ion beam. Then both bonding surfaces were allowed to face each other and press-bonded at 10 tonf to produce the composite substrate. As the functional substrates, lithium tantalate (LT) substrates and lithium niobate (LN) substrates were used. The bonding ability was evaluated from IR transmission images according to the following evaluation criteria: a composite substrate with a bonding area of 90% or more is rated as "best", a composite substrate with a bonding area of 80% or more and less than 90% is rated as "good ' and a composite substrate with a bond area of less than 80% is rated 'poor'.

4. Evaluation result

The cordierite-based sintered bodies of Experimental Examples 1 to 9 contained silicon nitride or silicon carbide and thus had more improved flexural strength and Young's moduli than the sintered body composed only of cordierite of Experimental Example 10. Specifically, the Young's moduli were 160 GPa or more and the four-point bending strength was 220 MPa or more. The cordierite-based sintered bodies of Experimental Examples 1 to 4 and 6 to 8 had a coefficient of thermal expansion of less than 2.4 ppm/°C (1.4 to 1.8 ppm/°C in Experimental Examples 1 to 4, in to which silicon nitride was added, and 1.8 to 2.3 ppm/°C in Experimental Examples 6 to 8 in which silicon carbide was added) at 40°C to 400°C. Although the coefficients of thermal expansion were slightly higher than those of the sintered bodies of cordierite alone of Experimental Example 10, the coefficients of thermal expansion were still kept at low levels. The cordierite-based sintered bodies of Experimental Examples 1 to 4 and 6 to 8 had an open porosity less than 0.1% and an average grain size of sintered grains of 1 μm or less, so that the mirror-finished surfaces had a small center-line average roughness Ra of 1.1 nm or less. Thus, when the cut slices of the cordierite-based sintered bodies of Experimental Examples 1 to 3, 6 and 7 were directly bonded to the functional substrates, the bonding ability was rated as “best” in which the bonding area is 90% or more. The bondability when the cut slices of the cordierite-based sintered bodies of Experimental Examples 4 and 8 were directly bonded to the functional substrates was rated as “good” in which the bond area is 80% or more and less than 90%. The fact that the number of pores was as small as three or less also contributes to the low center-line average roughness Ra of the mirror-finished surfaces. A reason why the mean grain size of sintered grains is 1 μm or less in Experimental Examples 1 to 9 is presumably that the sintering was carried out without using a sintering aid such as a rare earth element oxide.

Experimental examples 1 to 4 and 6 to 8 correspond to examples of the present invention. Experimental Examples 5, 9 and 10 correspond to Comparative Examples. The present invention is not limited to these experimental examples.

Claims (8)

A cordierite-based sintered body, the cordierite-based sintered body comprising: a coefficient of thermal expansion of less than 2.4 ppm/°C at 40°C to 400°C, an open porosity of 0.5% or less, and an average grain size of sintered grains of 1 µm or less, wherein the cordierite-based sintered body contains silicon nitride, and in the case where a polished surface is subjected to observation from a backscattered electron image and compositional analysis with an SEM, and then an area ratio of a cordierite phase to a silicon nitride phase is determined from a contrast ratio of the image, and is converted to a volume ratio of the sintered body, the cordierite phase is 60% to 90% by volume, and the silicon nitride phase is 10% to 40% by volume. A cordierite-based sintered body, the cordierite-based sintered body comprising: a coefficient of thermal expansion of less than 2.4 ppm/°C at 40°C to 400°C, an open porosity of 0.5% or less, and an average grain size of sintered grains of 1 µm or less, wherein the cordierite-based sintered body contains silicon carbide, and in the case where a polished surface is subjected to observation from a backscattered electron image and a composition analysis with an SEM, and then an area ratio of a cordierite phase to a silicon carbide phase is determined from a contrast ratio of the image and converted to a volume ratio of the sintered body, the cordierite phase is 70% to 90% by volume and the silicon carbide phase is 10% to 30% by volume. Cordierite-based sintered body claim 1 or 2 , wherein the number of pores having a maximum length of 1 µm or more and present in an area of 100 µm × 100 µm of a mirror-polished surface is 10 or less. Cordierite-based sintered body according to one of Claims 1 until 3 , wherein the cordierite-based sintered body has a Young's modulus of 160 GPa or more. Cordierite-based sintered body according to one of Claims 1 until 4 , wherein the cordierite-based sintered body has a four-point bending strength of 220 MPa or more. Cordierite-based sintered body according to one of Claims 1 until 5 , wherein a mirror-polished surface has a center-line average roughness Ra of 1.5 nm or less. A method for producing a cordierite-based sintered body according to any one of Claims 1 or 2 A composition comprising the steps of: (a) preparing a raw material powder mixture by mixing 60% to 90% by volume of a cordierite powder having an average particle diameter of 0.1 to 1 µm and 10% to 40% by volume of a silicon nitride powder an average particle diameter of 0.1 to 1 µm in a total amount of 100% by volume or by mixing 70% to 90% by volume of a cordierite powder having an average particle diameter of 0.1 to 1 µm and 10% to 30% by volume % of a silicon carbide powder having an average particle diameter of 0.1 to 1 µm in a total amount of 100% by volume; and (b) preparing a cordierite-based sintered body by forming the raw material powder mixture into a compact having a predetermined shape and subjecting the compact to hot pressing at a compacting pressure of 20 to 300 kgf/cm ² and a firing temperature of 1350°C to 1450°C °C Use of the cordierite-based sintered body according to one of Claims 1 until 6 as a supporting substrate in a composite substrate.

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